This invention relates to seat assemblies for motor vehicles, and more particularly to seat adjusters for vehicle seat assemblies.
Seat adjusters for vehicle seat assemblies include track mechanisms which support the vehicle seat at each side of the seat and provide for adjustment in the positioning of the seat. The track mechanisms include upper and lower tracks. The lower track is fixed to the floor of the vehicle and the upper track is supported on the lower track and adapted for manual movement, or to be motor driven, relative to the lower track to provide fore-and-aft adjustment in the position of the seat. In addition, many seat adjusters also provide up and down adjustment and independent tilting of the front and rear of the seat and tilting of the back to accommodate a wide variety of drivers.
To facilitate horizontal movement and provide smooth adjustment, bearing assemblies including roller bearings have been proposed for the track mechanisms of seat adjusters. The roller bearings provide a smooth, precise guide between the moving upper track and the fixed lower track. However, under load, the tracks tend to bow at their centers so that there is less roller to track contact at the center of the rollers, decreasing lubricant effectiveness and resulting in accelerated wearing of the tracks, particularly at the edges of the rollers. This is particularly true when the sliding interface includes steel roller bearings and the tracks are made of a light weight material such as aluminum. In recent years, tracks made of lightweight metal, such aluminum, have been introduced. The use of aluminum provides significant weight savings which can be about 10% less than a conventional steel track system.
Another consideration is that in most seating adjusters presently available, the mating moving parts are lubricated with grease. The use of grease on mating parts can result in the transfer of grease to occupants of the passenger compartment of the vehicle if a pant cuff or coat hem should inadvertently contact a greased surface. Various types of dry lubricants are known such as polytetrafluoroethylene, molybdenum disulfide, graphite, and the like. Although polytetrafluoroethylene has a low coefficient of friction, its use is limited to load conditions that are substantially lower than those experienced in the operation of seat track mechanisms. Molybdenum disulfide is suitable for use under high load conditions, but the high hardness binder in the coating promotes roller squeaking due to the roller sliding over the hard dry lubricant surface, resulting in undesirable noise. Moreover, it has been found that in a mixture of polytetrafluoroethylene and molybdenum disulfide, there is a weakening of the binder of the molybdenum disulfide which promotes build up of dry lubricant flakes that prevent proper travel of the steel rollers in the tracks.
In most power seat adjusters, horizontal, vertical and recliner adjustment is provided by screw drive mechanisms which include a drive screw and a drive nut. The drive nuts of horizontal, vertical and recliner screw drive mechanisms of currently available seat adjusters are formed from metal tubes. Although the drive nuts may include a threaded plastic insert for receiving the associated drive screw to eliminate the metal-to-metal connection between the drive screw and the drive nut, known screw drive mechanisms provide metal-to-metal connections between the vertical drive nut and the coupling mechanism that connects the drive nut to the seat support and between the recliner drive nut and the seat back frame. This metal-to-metal connection results in noise, higher friction and reduced efficiency caused by relative movement between these parts during adjustment in the vertical positioning of the seat or in adjustment of the inclination of the seat back.
In power seat adjusters, adjustment of the horizontal positioning of the vehicle seat assembly is provided using a direct drive for one track mechanism, such as the outboard track, and a slave drive for the inboard track. The drive screw of the horizontal drive mechanism of the outboard track is connected through a transmission unit directly to the motor and the drive screw mechanism of the inboard track is connected to the drive motor through a transmission unit and a flexible shaft. In known power seat adjusters, travel limit stops are provided both on the master drive screw drive mechanism and on the slave screw drive mechanism. Due to manufacturing tolerances, if the slave drive mechanism reaches its end of travel stop before the master drive mechanism reaches its end of travel stop, the drive motor can continue to drive the master drive mechanism because the flexible shaft that connects the slave drive mechanism to the motor can twist, resulting in over travel in the outboard track section after the inboard track section has reached its end of travel stop, a condition referred to as horizontal matchboxing. A further consideration is that because there is high energy running into a stop, the screw drive mechanisms jam when they reach their two extreme stop positions and the drive screw mechanisms lock up. Consequently, a greater force is required to overcome frictional forces to reverse the movement drive screws in moving the drive screw away from its extreme stop positions.